If the vehicle is AWD, then it is in 4wd all of the time and you don't need to turn it on. THe vehicle has 2 differentials, a front and a rear. The item in the center (near the trans) is called a transfercase. It is the part that sends the power to both front and rear.
As far as traction, the tire with the least resistance will spin first. However, at least one front and one rear tire will spin, so if you have the front on ice and the rear has traction, you will still go.
Let me know if you have other questions.
Tuesday, October 27th, 2009 AT 7:59 AM